Data storage disc systems are widely used because of the rapid data storage and acquisition times possible with such systems. For reasons of economy and simplicity, such systems generally have disc drives which spin their data storage discs at a constant angular velocity. If the disc drive write clock has a single fixed frequency to provide a single fixed writing speed, the maximum possible data storage density is established on the smallest radius data track, since the larger radii data tracks have a greater track circumference with an identical angular velocity. Thus, recording density with a constant writing speed is less than optimum for such systems, particularly if the ratio of smallest to largest data track radii is large.
Since it is generally not feasible to operate disc drives in a constant linear track velocity mode, it is desirable to vary the write clock frequency to alter the data writing speed, so that writing speeds increase with increasing data track radius. Such variable frequency writing systems can often approach the optimum possible data storage density for a given disc and signal transducer combination.
However, it is not feasible to use a constantly variable frequency writing system having a frequency which is proportional to radius because of the necessity for supplying an integral number of data track sectors and servo frames if consistent sector lengths are to be maintained. Some disc drive systems include write clocks which change frequency only for groups of tracks, or bands, to provide an integral number of track sectors and servo frames for each band. In this way, a minimum selected number of sectors with a selected number of servo frames per sector for each sector is established for the innermost band with the smallest radii data tracks. The sector frequency and write clock frequency is increased for more outwardly bands with increased data track radii which can accommodate a larger integral number of sectors.
It is advantageous for disc drive systems to use one side of a disc specifically for positioning and tracking information, while using the other side and other discs for data storage tracks. The side of the disc used for positioning and tracking information is called the "dedicated" surface, and it has a series of servo tracks written on it which correspond in number and radii to the data tracks written on the discs. The servo tracks include accurate circumferential and radial positioning information, typically with twenty to thirty thousand position samples per revolution. This provides exceptionally high seek and timing performance, since a special servo head is mechanically coupled to the read/write heads which track the corresponding data tracks on the discs.
It is also advantageous for such disc drive systems to have tracking information embedded in "half-tracks" between the servo tracks of the dedicated surface. This embedded tracking information is typically in the form of alternating high frequency bursts on each side of each servo track, to provide an accurate off-center tracking indication which may be used for tracking error correction. Typically, one such bursts sample is embedded on each side of each servo track for each track sector.
The accurate position information recorded on the servo tracks of such discs with a dedicated surface is ideal for write clock frequency generation, since the ratio of the frequency of the position information to the disc rotational frequency is constant. This relationship is constant from track to track, regardless of radius. The multiple frequency write clock systems used to provide fixed length, variable frequency sector recording to maximimize recording density as described above do not use such an embedded and dedicated servo configuration. This is because of the difficulty in generating the multiple frequencies required for the write clock that are at the same time all synchronized with the positioning and tracking information on the dedicated surface.